Electron-transfer rates reaction center

Electron transfer between metal centers can alter the course of reaction in several ways (46). Thermal excitation may create especially reactive electron holes on the oxide surface, causing reductant molecules to be consumed at the surface at a higher rate. More importantly, electrons deposited on surface sites by organic reductants may be transferred to metal centers within the bulk oxide (47). This returns the surface site to its original oxidation state, allowing further reaction with reductant molecules to occur without release of reduced metal ions. Electron transfer between metal centers may therefore cause changes in bulk oxide composition and delay the onset of dissolution. 

One of the enigmatic problems of photsynthesis is the drastic difference between the rate of photelectron transfer in the active (M) and inactive branches of bacterial reaction centers. The quantum mechanical calculation (Kolbasov and Scherz, 2000) showed that the square of electronic matrix element VA2 for the electron transfer from the excited primary donor, P, to bacteriochlorophyl in the active brunch is larger by three order of magnitude than that in the inactive part Vb2. Therefore, the electron transfer rate in the RC inactive L-brunch should be essentially slower than that in the M-brunch. 

Reaction center experiments show that the electron transfer rate is not very sensitive to the detailed ehemieal nature of the donor and aeeeptor. Thus ubiquinone ean be replace by naphthoquinones, anthraquinones, fluorenones and even dinitrobenzenes (Wamcke and Dutton, 1993 Moser et al., 1992), yet all these speeies can still be embraced by a single broad Gaussian free energy dependenee. 

Chains also contribute to the robustness of natural electron transfer protein design because the close spacing between successive redox centers means that the driving force of the reaction can usually vary widely with relatively little effect on the overall electron transfer rate through the protein. Indeed, many naturally occurring chains have uphill electron transfer steps of hundreds of meV. 

Gudowska-Nowak, E., 1994, Effects of heterogeneity on relaxation dynamics and electron transfer rates in photosynthetic reaction centers. J. Phys. Chem., 98 5257n 5264. 

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